1. Field of the Invention
The present invention relates generally to a method for fabricating a copper/indium/gallium/selenium (CIGS) solar cell, and more particularly, to a method for preparing a sol-gel solution for fabricating a CIGS solar cell.
2. The Prior Arts
Sources of fossil fuel had been mined and non-renewably consumed for many years, and are almost exhausted from the earth. It is a critical concern for the human being to find out reliable alternative energy sources for even the basic demand, survival. Biomass energy, geothermal energy, wind energy, and nuclear energy are all in consideration. However, when further in view of factors of reliability, security, and environment protection, none of them can be comparable with solar energy taken from the sunlight radiation. Almost everywhere of the earth can be illuminated by the sunlight, and the sunlight can be received and converted into electric energy without producing any contaminant. Therefore, solar energy is so far the cleanest alternative energy source.
A solar cell is a device for converting the sunlight energy into electric energy which can be more conveniently used. There are many kinds of solar cells developed and produced for satisfying different demands. Among all of these kinds, CIGS solar cell having the advantages of a higher absorbing efficiency and a higher photoelectric conversion efficiency is paid with more attention thereto.
In general, the CIGS solar cell is derived from a copper/indium/selenium (CIS) solar cell. The CIS solar cell includes CuInSe2 as a main ingredient. CuInSe2 is a semiconductor with a direct bandgap, especially having a very high absorbance. The forbidden bandwidth (Eg) of CuInSe2 is 1 eV which is less than the range 1.4 to 1.5 eV which is believed as most suitable for a solar cell. As such, Cu(InGa)Se2 having a higher forbidden bandwidth, Eg=1.6 eV, which is known as a CIGS polycrystalline material is prepared for improvement. The CIGS solar cell can achieve a high photoelectric conversion efficiency up to 19%, and a CIGS solar cell module can correspondingly achieve a relatively high photoelectric conversion efficiency up to 13%. Corresponding to different contents of the indium and gallium ingredients contained therein, the light absorbing range may extend from 1.02 eV to 1.68 eV, and thus achieving a high light absorbing efficiency up to 105 cm−1. As such, even a CIGS thin film having a thickness less than 1 μm can absorb more than 99% photons of the sunlight illuminated thereon.
FIG. 1 is a schematic diagram illustrating a conventional CIGS solar cell 1. Referring to FIG. 1, the CIGS solar cell 1 typically includes a glass substrate 10, a back electrode layer 20, a CIGS light absorbing layer 30, a buffer layer 40, and a transparent electrode layer 50. The back electrode layer 20 is provided for electric conduction, and is typically made of molybdenum. The CIGS light absorbing layer 30 is a p-type semiconductor layer, adapted for absorbing light. The buffer layer 40 is an n-type semiconductor layer, and typically made of cadmium sulfide (CdS). The transparent electrode layer 50 is typically made of aluminum zinc oxide (AZO), indium zinc oxide (IZO), or indium tin oxide (ITO), which has a high light transparency and a high electric conductivity. As shown in FIG. 1, a sunlight L is downwardly incident to the CIGS solar cell 1. The sunlight L enters the transparent electrode layer 50, and then passes through the buffer layer 40, and reaches the CIGS light absorbing layer 30. The CIGS light absorbing layer 30 absorbs the sunlight L and produces electron hole pairs having electric potential energy. The transparent electrode layer 50 and the back electrode layer 20 then output the electric potential for power supplying.
FIG. 2 is a flow chart illustrating a process for fabricating the conventional CIGS solar cell. Referring to FIG. 2, first at step S10, a magnetron sputtering process is conducted in which a molybdenum target is bombarded for depositing a back electrode layer having a thickness about 1 mm onto a glass substrate. Then, at step S20, a CIGS light absorbing layer having a thickness about 2 mm is adaptively deposited onto the back electrode layer. Then, at step S30, a chemical bath deposition (CBD) process is conducted for configuring a buffer layer, e.g., CdS. And finally, at step S40, a sputtering process or a chemical vapor deposition (CVD) process is conducted for providing a transparent electrode, thus configuring the CIGS solar cell as shown in FIG. 1.
Typically, evaporation deposition, sputtering deposition, electrochemical deposition, and ink coating are often selected for configuring the CIGS light absorbing layer at step S20 during the above-illustrated process for fabricating the conventional CIGS solar cell. However, each of the evaporation deposition, the sputtering deposition, and the electrochemical deposition involves a vacuum processing which requires expensive equipment investment. As to the ink coating technology, it is a non-vacuum technology developed by International Solar Electric Technology Inc. (ISET). According to the ink coating technology, metal or oxide nanoparticles are prepared by nanotechnology, and are then mixed with a suitable solvent thus forming a gelatinous mixture. Then, the gelatinous mixture is provided onto the molybdenum layer to configure the CIGS light absorbing layer by for example an ink process. In such a way, the ink coating technology can save much fabrication cost.
Unfortunately, the metal or oxide nanoparticles required for the ink coating technology are not easy to prepare, and the solvent for forming the gelatinous mixture is inconvenient to compound. Therefore, the ink coating technology lacks reliable source of raw material.
As such, a method for preparing a gelatinous mixture of metal particles and a solvent is desired for uniformly mixing the metal particles and the solvent with an adaptive mixing process thus providing a solution of the conventional technology.